Estimation of tiger densities in India using photographic captures and recaptures

Ecology, Dec, 1998 by K. Ullas Karanth, James D. Nichols

DISCUSSION

Applicability of the photographic capture - recapture method

In our short-duration surveys, camera traps were mounted in the open and suffered some damage from occasional attacks by elephants and rhinos, and, on three occasions, by tigers. At places where animal damage or theft by humans is a significant factor, it may be necessary to provide robust protective structures for the camera traps.

Our study showed that photographic capture-recapture studies offer a practical and reasonable approach to estimating the abundance of tiger populations. The estimated average capture probabilities (0.11-0.22 per occasion) were fairly good, and certainly much higher [TABULAR DATA FOR TABLE 4 OMITTED] than capture or detection probabilities resulting from any other tiger sampling methods of which we are aware. The estimated probabilities of missing an animal [Mathematical Expression Omitted] were [less than]0.25, and overall capture probabilities were very high. Despite the reasonable capture probabilities, however, the total numbers of animals caught were low ([less than]30). These low numbers are expected to limit our ability to investigate sources of variation in capture probability and, thus, to select the most appropriate model for estimating abundance (e.g., Otis et al. 1978, White et al. 1982). We dealt with this limitation by basing our density estimates on the [M.sub.h] estimator, which is the most robust of the estimators considered by Otis et al. (1978). Of course, we would still like to base our estimates on larger numbers of animals, but our small samples were a natural consequence of studying animals at such low densities. Despite the limitations of our approach imposed by low tiger densities, we believe that camera-trapping is superior to every other method that we have considered for estimating tiger abundance and density.

As noted by Karanth (1995), we found that tiger cubs (age [less than] 1 yr) had low capture probabilities, with no animals in this age class being captured at Kaziranga, and only one capture each in Kanha and Nagarahole. The only exception was at Pench, where one particular litter was caught four times. Moreover, although we knew from direct observations that several cubs existed at Nagarahole, none of them was camera-trapped. Deliberate avoidance of traps and other behavioral differences between young and adults usually appear to result in cubs having extremely low capture probabilities. Therefore, estimating population size for tiger cubs [TABULAR DATA FOR TABLE 5 OMITTED] may require impractically high trapping effort. However, the numbers of cubs in an area can be estimated empirically from the number of resident females with litters, based on ancillary observations (Schaller 1967, McDougal 1977, Sunquist 1981, Smith 1993) or, theoretically, using demographic models (Kenny et al. 1995, Karanth and Stith, 1998).

Our approach to estimating tiger densities may also be useful for estimating abundance and density of a number of other secretive animal species that are individually recognizable based on natural markings. In many species with no natural markings, it may be possible to use photographic recaptures following an initial physical capture and artificial marking. We note that the approach can be used, with some modification, when only a subset of the population is potentially identifiable. There also appears to be potential for using long-term camera-trapping surveys to estimate additional population parameters such as survival, mortality, recruitment, and dispersal rates for tigers (and other individually recognizable animals), by applying open capture-recapture models that are currently available (Seber 1982, Pollock et al. 1990, Lebreton ctal. 1992).